Arthur Darvill and Joanna Christie are adequate, and “Once” is a treasure. Photo: Joan Marcus

There are two ways to score a Broadway hit: Load up on name stars and make a mint during a limited run, or bank on good bones that can get through years of cast changes.

Anything involving Hugh Jackman or Denzel Washington belongs to the first category. The second boasts the likes of “Jersey Boys,” “The Phantom of the Opera,” “The Lion King” and “The Book of Mormon” — sorry, elders Josh Gad and Andrew Rannells, but nobody ever paid premium prices to specifically see you.

And then there’s “Once,” which is an interesting hybrid: a hit show blessed with terrific assets — John Tiffany’s brilliant direction, Glen Hansard and Markéta Irglová’s swoony songs — but where casting also plays a key part.

Sadly, this last point is driven home by the weakness of the new leads, Arthur Darvill and Joanna Christie. They’re adequate, but you don’t buy the romantic yearning their characters — Dublin musicians who go by the names Guy and Girl — are meant to share.

Based on the movie of the same title, “Once” has done tremendously so far without marquee names, winning Tonys and playing to near-capacity rooms. But as a love story, it does require actors with chemistry. Not only that, but they have to elevate characters who can be — let’s face it — kind of annoying.

Charming original stars Steve Kazee and Cristin Milioti helped us overlook the fact that he played a passive mope and she was saddled with a Czech version of the dreaded “manic pixie dream girl” archetype.

Now the characters’ drawbacks take over, because Darvill and Christie largely lack personality. You’ve got to wonder why the show’s producers had to import them from the UK — if there’s one thing America has in spades, it’s technically adept but bland performers.

Christie actually does marginally better with Girl, drawing laughs at all the right times with her deadpan comebacks and playing the piano nimbly. But Darvill — best known for a recurring role on “Doctor Who” and currently in “Broadchurch” — lacks Kazee’s uncanny ability to turn plaintiveness into frustrated passion.

Luckily, the show’s built-in features still provide a strong foundation. The sterling ensemble, which doubles as onstage orchestra, is anchored by solid performances from original cast members Paul Whitty (as bearded quirkmeister Billy), David Patrick Kelly (as Guy’s solemn but loving Da) and Lucas Papaelias (as soap-opera-loving Svec).

And then there’s the collaboration between director Tiffany and choreographer Steven Hoggett. Rarely has movement felt so poetically organic in a musical. It’s not even dance anymore, but life itself. When music and bodies fall into sync, “Once” reclaims its title as Broadway’s most idiosyncratic treasure.